The present invention relates to controlling powertrain lash crossing and in particular to controlling lash crossing for a modular hybrid transmission.
Lash crossing of a powertrain is when the powertrain transitions between producing a positive torque and receiving a negative torque. At a zero torque transition point between the positive and negative torques the powertrain produces lash when powertrain components, including transmission gears, separate when torque is removed at the transition point before the components resume contact when torque is reapplied. Lash may produce a clunk noise or bump that is perceivable to a vehicle occupant. Lash contributes to noise, vibration, and harshness that degrades driveability of a vehicle using the powertrain.
Timing of lash crossing is controllable by a powertrain control system. During deceleration of the vehicle using a conventional powertrain, the lash crossing is controlled to occur when a driveshaft in the powertrain has slowed to an idle speed of an internal combustion engine (approximately 750 RPM). Were the driveshaft allowed to slow below the idle speed, the engine would stall. Additionally, the vehicle typically has hydraulic systems powered by the engine. The hydraulic systems have been designed to operate with a minimum operating speed equal to the idle speed because, due to stalling, the conventional powertrain does not operate below the idle speed.
Alternatively, the vehicle may use a hybrid powertrain. Typically, for the hybrid powertrain operating in an electric only mode, lash crossing is controlled to occur when the driveshaft has slowed to the idle speed of the engine in the conventional powertrain even though an electric machine in the hybrid powertrain does not have a stall limitation. By lash crossing the machine at the idle speed, hydraulic systems designed for use with the conventional powertrain can be used with the hybrid powertrain as well. Lash crossing for a hybrid powertrain continues to be at the idle speed of the engine despite the availability of hydraulic systems designed to have minimum operating speeds lower than the idle speed.
However, lash crossing the hybrid powertrain at the engine idle speed may reduce powertrain efficiency. Lash crossing the hybrid powertrain at the idle speed entails the electric machine being operated at the idle speed, which may deplete an electrical power source. A regenerative braking capacity may also be reduced.